Review: Geological Time Scale

• Geologic time is divided hierarchically into eons → eras → periods → epochs.
  
• In this course, we focus on the Precambrian, especially the Neoproterozoic.

Source: https://www.geosociety.org/

Major Time Units

• Precambrian (≈4600–541 Ma)
  • Covers ~88% of Earth history
    
  • Time of early crust formation and supercontinent cycles
• Proterozoic Eon (2500–541 Ma)
  • Stabilization and growth of continental crust
    
  • Multiple supercontinent assemblies and breakups

Neoproterozoic Era (1000–541 Ma)

Source: https://www.geosociety.org/

• Tonian (1000–720 Ma)
  • Initial breakup of Rodinia
    
  • Early ocean opening and arc magmatism
• Cryogenian (720–635 Ma)
  • Widespread glaciations (“Snowball Earth”)
    
  • Major development of juvenile volcanic arcs
• Ediacaran (635–541 Ma)
  • Arc collision, granitoid intrusion
    
  • Assembly of Gondwana
    
  • Final stabilization of the Arabian Shield

Review: Main Plate Tectonics Terminology

Plate tectonics explains the movement of rigid lithospheric plates and the geological processes at their boundaries.

Fundamental Concepts

• Lithosphere
  Rigid outer shell of the Earth (crust + uppermost mantle)

• Asthenosphere
  Hot, weak upper mantle beneath the lithosphere that allows plate motion

• Plate
  Large, rigid slab of lithosphere moving over the asthenosphere

Plate Boundaries

• Divergent boundary
  Plates move apart; new crust forms
  Example: Red Sea rift

• Convergent boundary
  Plates move toward each other

  • Ocean–ocean → island arcs
    
  • Ocean–continent → continental arcs
    
  • Continent–continent → orogens

• Transform boundary
  Plates slide past one another
  Example: Dead Sea Transform

Subduction-System Terminology

• Trench
  Deep-sea feature marking the surface expression of subduction

• Forearc
  Region between the trench and the volcanic arc

  • Characterized by accretionary prisms, forearc basins
    
  • Dominated by sedimentation and deformation

• Volcanic arc
  Chain of volcanoes and plutons formed above a subducting slab

• Backarc
  Region behind the volcanic arc, away from the trench

  • May experience extension and backarc spreading
    
  • Common site of basin formation and volcanism

Key Tectonic Processes

• Subduction
  Sinking of dense oceanic lithosphere into the mantle

• Arc magmatism
  Volcanism and plutonism above subduction zones

• Orogeny
  Mountain building through collision and crustal thickening

• Rifting
  Stretching and thinning of continental lithosphere

Crustal Architecture Terms

• Craton
  Stable continental core

• Shield
  Exposed Precambrian basement of a craton

• Terrane
  Fault-bounded crustal block with a distinct geologic history

• Suture zone
  Remnant of a closed ocean between colliding plates

• Ophiolite
  Fragment of oceanic lithosphere emplaced onto continental crust

What is the Arabian Shield?

• The Arabian Shield is an extensive exposure of Precambrian crystalline basement in western Saudi Arabia.
• It consists mainly of volcanosedimentary rocks intruded by large volumes of diorite, tonalite, granodiorite, and granite.
• Most of the shield formed during the Neoproterozoic (1000–542 Ma).
• The core geological history spans ~850–550 Ma, when oceanic crust evolved into continental crust.

Location map of the Arabian Shield, Source: Kashghari et al. (2025)

Arabian Shield Overview

Image generated by NotebookLM

Supercontinents and the Arabian Shield

What is a Supercontinent?

• A supercontinent is an unusually large landmass formed by the collision and amalgamation of multiple continents and cratons.
  
• Supercontinents form and break apart through the supercontinent cycle (Wilson Cycle).

General cycle: 1. Continental breakup and rifting
2. Ocean basin formation
3. Subduction and arc magmatism
4. Continental collision and assembly
5. Erosion and stabilization → renewed breakup

Major Supercontinents (Precambrian–Phanerozoic)

• Rodinia (≈1.3–0.75 Ga) – broke apart in the early Neoproterozoic
• Gondwana (assembled by ≈530 Ma) – incorporated Africa, Arabia, South America, Antarctica, India, Australia
• Pangea (≈320–180 Ma) – later Phanerozoic supercontinent

Arabian Shield in the Supercontinent Cycle

• The Arabian Shield formed after the breakup of Rodinia.
• It developed as juvenile oceanic arcs above subduction zones.
• Multiple arcs collided and accreted during the Neoproterozoic.
• These collisions contributed to the assembly of Gondwana.

Global Context

• The Arabian Shield is the northern part of the Arabian–Nubian Shield.
• It represents the northern termination of the East African Orogen.
• Together, these belts record one of the largest Neoproterozoic crust-forming events on Earth.